Residual Stress Gradient Research Articles

A repulsive magnetic force driven translation micromirror

This paper presents a repulsive magnetic force driven micromirror with large displacement and high surface quality which well solves the limitation of the previous design, i.e. large variation in translation starting position and low repeatability, caused by the touching points between the moving film and substrate before and in operation. The new design utilizes a driving mechanism, i.e. permanent magnet ring above and electromagnet underneath the moving film, to lift the moving film from touching the substrate and generate a repulsive magnetic force (instead of attractive force in the previous design) to push the moving film up and away from the substrate for translation. Due to the touching, the previous design has to pre-oscillate for 20–30 min at 1 Hz before usage (after resting for a few hours) to reduce the starting position variation from ~15 µm to 3–4 µm. Even after the pre-oscillation, the repeatability is still low, which is 14.2% because of the touching in operation. In the design presented in this paper, the touching between the moving film and the substrate is completely eliminated before and in operation. As a result, the starting position of the translating mirror is constant each time and the repeatability is <1%. In addition, this design does not need the residual stress gradient to curve up the moving film. The maximum displacement of 144 µm can be achieved when 140 mA current is applied on the electromagnet. As an application, the micromirror is used as the movable mirror in a Michelson interferometer to measure the wavelength of a laser beam. The result shows a measurement accuracy of 2.19% for a 532 nm laser beam.

Effect of hatch length on the development of microstructure, texture and residual stresses in selective laser melted superalloy Inconel 718

In the present study, samples fabricated by varying the deposition hatch length during selective laser melting of nickel based superalloy Inconel 718 were investigated. Microstructure and texture of these samples was characterized using scanning electron microscopy, combined with electron back-scattered diffraction, and residual stress assessment, using neutron diffraction method. Textured columnar grains oriented along the sample building direction were observed in the shorter hatch length processed sample. A ten-fold increase in the hatch length reduced the texture intensity by a factor of two attributed to the formation of finer grains in the longer hatch length sample. Larger gradients of transverse residual stress in the longer hatch length sample were also observed. Along the build direction, compressive stresses in the shorter hatch length and negligible stresses for the longer hatch length specimen were observed. Changes to the temperature gradient (G) in response to the hatch length variation, influenced the G to growth rate (R) ratio and the product G×R, in agreement with the microstructures and textures formed. For the residual stress development, geometry of the part also played an important role. In summary, tailored isotropy could be induced in Inconel 718 by a careful selection of parameters during selective laser melting.

Study of residual stresses in Ti-7Al using theory and experiments

Finite element simulations are carried out to follow the evolution of residual stresses in Ti-7Al (α-hcp) alloy, as developed under an applied stress gradient. A model built upon phenomenological mesoscopic field dislocation mechanics is employed to simulate the deformation behavior. Model predictions are validated with results generated from high energy X-ray diffraction experiments using synchrotron radiation. These experiments provide for important simulation input, viz. grain positions and orientations, and strain rate sensitivities of the prismatic and basal slip systems of Ti-7Al. X-ray diffraction data obtained from individual grains enabled calculation of strain rate sensitivities of the prismatic and basal slip systems and the values are estimated as ∼ 0.04 and ∼ 0.02 respectively. Residual stresses at the length scale of individual grains and subgrains are successfully predicted and validated against experimental data. A key achievement of the present work is the measurement and simulation of residual stress gradients within individual grains. Conclusions from this work are that grains deform mainly via prismatic slip, and accurate characterization of rate-sensitivity is needed to model the development of grain-level residual stresses.

Non-Destructive Determination of Residual Stress Depth Profiles of Hybrid Components by Energy Dispersive Residual Stress Measurement

Through the combination of two or more materials to one compound, for example high-strength steel and aluminum, hybrid massive components can be manufactured, whose properties are specially adapted to the respective application. One of the challenges is the joining zone which is influenced by machining induced residual stresses. In order to examine the residual stress modifications by the machining process and in addition to analyze the influence of these residual stress gradients on the lifespan of hybrid components a non-destructive method of measuring depth-resolved residual stress is necessary. Therefore, an innovative energy dispersive X-ray measurement technique is used in the collaborative research center 1153 (CRC 1153). In this study the suitability of the method is examined by comparing the results with the angle dispersive method both in machined front surface of mono materials and hybrid shafts. A parametrical study shows the possibility to get greater depth information by variation of the measuring parameters Bragg angle, tilting angle, collimator and current. In addition, the results of the energy dispersive method combined with electrolytic removal is shown. Based on these results the evaluation of the reliability and reproducibility of energy dispersive residual stress measurements is completed.

Mechanical and thermal properties of double-layer and triple-layer thermal barrier coatings with different ceramic top coats onto polyimide matrix composite

Abstract The present study has discussed the deposited thermal barrier coatings of Cu-Sn/mullite and Cu-Sn/NiCrAlY/YSZ onto a carbon-fiber reinforced bismaleimide (BMI) matrix composite. The thicknesses of the metallic bonding layers and ceramic top coats were 100 µm and 300 µm, respectively. The microstructural, mechanical and thermal properties of the TBC systems were evaluated through x-ray diffraction (XRD), scanning electron microscopy (SEM), tensile adhesion strength testing (TAT), micro-hardness tests, roughness measurement, thermal shock testing and laser flash analysis (LFA). A comparison of the microstructure of the ceramic top coats showed that the YSZ had a homogeneous structure with high crystallinity while the mullite exhibited a multi-phase structure containing mullite, alumina (γ-Al2O3) and amorphous phases. The mullite layer recorded higher residual stress than that the YSZ layer because of its higher Young's modulus and the transformation of the mullite phase to alumina and amorphous phases during deposition. The adhesion strength of the sample with a triple-layer coating was greater than that with a double-layer coating because it exhibited a milder residual stress gradient across the coating. The thermal shock resistance of the triple-layer coated sample was 1.5-fold greater than that of the double-layer coated sample. This could be due to the existence of an oxidation-resistant layer of NiCrAlY and smaller differences in the coefficients of thermal expansion between coating layers in the triple-layer coating system. Thermal conductivity of the triple-layer coating sample was much less than that of the double-layer coating which can increase the operating temperature of the polymeric substrate to a higher value.

High resolution residual stress gradient characterization in W/TiN-stack on Si(100): Correlating in-plane stress and grain size distributions in W sublayer

Residual stress gradient characterization by the ion beam layer removal method (ILR), using a milling step of 10nm, was applied to W/TiN stacks processed on thermal SiO2-insulated standard silicon wafers. The stress profiles indicate a pronounced stress gradient with high tensile, as well as compressive stress concentrations in polycrystalline W and amorphous TiN sublayers ranging between 3.5 and −4GPa. Electron backscatter diffraction shows that the W sublayer exhibits zone T microstructure with nano-sized crystallites in the nucleation region at the interface to TiN and above that columnar or V-shaped grain morphology typical for competitive grain growth. In the W sublayer, the stress distributions correlate well with this in-plane crystallite size distribution on the base of a Hall-Petch mechanism, reaching a tensile maximum in the transition region between the nucleation layer and the region with columnar microstructure.

Origin of in Situ Domain Formation of Heavily Nb-Doped Pb(Zr,Ti)O3 Thin Films Sputtered on Ir/TiW/SiO2/Si Substrates for Mobile Sensor Applications.

High-quality piezoelectric thin films have recently been in demand for mobile sensor applications. An investigation was conducted to understand the improvements in the piezoelectric and imprint characteristics of heavily Nb-doped lead zirconate titanate thin films with an extensive range of Nb content (up to 14 mol %) beyond the typical solid solubility limit of Nb. The positive effects produced by the unusual doping of Nb were realized by utilizing an in situ sputtering process that did not require a subsequent annealing and poling procedure. An enhanced piezoelectric coefficient, -e31, of -12.87 C/m2 and a stronger shift in the coercive field, Ec,shift, of ∼20 kV/cm, which are ideally useful for mobile sensor applications, were obtained for the 12 mol % Nb-doped films deposited on nonconventional buffer electrodes of Ir/TiW. The reduced oxygen vacancy concentration and preferred domain orientation with a stronger piezoresponse induced by the Nb donor doping contributed to the enhancement of the piezoelectric properties. Potential defect dipoles aligned by a residual stress gradient along columnar structures seemed to induce an internal electric field in the Nb-doped films, leading to the preferred domain orientation, as well as the strong imprint behavior due to a clamping of domain walls.

Design, simulation and analysis of a digital RF MEMS varactor using thick SU-8 polymer

This paper presents the design, simulation and analysis of a 5-bit RF MEMS digital variable capacitor using thick SU-8 polymer to achieve both high capacitance ratio (varvec{C}_{varvec{r}}) and Q-factor. The proposed varactor design consists of five capacitive shunt switches loaded over a co-planar waveguide (CPW) line. This results 32 capacitance steps ranging from 102.23 fF to 3.57 pF (varvec{C}_{varvec{r}} = 35). The high capacitance ratio of the varactor is achieved by implementing SU-8 polymer as the base structure of the capacitor due to its low dielectric constant (varvec{varepsilon}_{varvec{r}} = 4) hence reducing the overall minimum capacitance (varvec{C}_{{varvec{min}}}). The optimization of the SU-8 thickness has been carried out to get the optimum capacitance ratio and Q-factor. Moreover, the inclusion of semi-elliptical slot at the ground of CPW and variable sizes of CPW line further reduce the parasitic capacitance. To improve the reliability of the variable capacitor design, a new aluminium (Al) stopper design is used to prevent contact between the beams and the pull-down electrodes. Mechanical simulations and analysis have been carried out to investigate the effects of in-plane residual stress and stress gradient on the spring constant and the initial displacement of the proposed horizontal truss fixed–fixed beam structure. Compared to the conventional doubly clamped solid beam design, the proposed beam has less sensitivity to in-plane residual stress. This makes the pull in voltage to be less affected by the stress due to the fabrication. The simulated pull-in and lift-off voltages are 30 and 25 V respectively while the estimated switching time for the beam is 4.64 μs. The overall size of the varactor is 740 μm × 653 μm. The proposed RF MEMS varactor could be integrated in reconfigurable filters, phase shifters and matching networks targeting future multi-standard wireless communication systems.

Through-Thickness Deformation Gradient in a Part-Pilgered Zirconium Tube: Experimental Measurements and Numerical Validation

Through-thickness microstructure development and residual stress evolution were explored in a part-pilgered Zircaloy-4 tube. Clear gradients in microstructure and residual stress were experimentally established at different locations. Such locations were considered along the axial length and also across the wall thickness. These were naturally subjected to different von Mises effective strains and were reflected on strain/microstructural gradients. The deformation gradients were also simulated with a three-dimensional elastoplastic finite element model. The model used both isotropic and anisotropic yielding. Though deformation gradients were similar with both yielding criteria, the anisotropic yielding provided a better match with experimental residual stress gradients.

Residual stresses induced by laser welding process in the case of a dual-phase steel DP600: Simulation and experimental approaches

This study aimed at characterizing the residual stresses distribution of a DP600 undergoing a laser beam welding. The residual stresses in the ferritic phase have been experimentally determined by the use of the neutron diffraction technique. The results confirmed a gradient of residual stresses among different zones both on the top and below surfaces but also through the thickness of the fusion zone. Low compressive stresses were observed in the BM (Base metal) close to the HAZ zone (heat affected zone) whereas high tensile stresses were observed in the FZ (fusion zone). Two numerical modelling strategies were conducted: first with elastic plastic model (EP) and then with a visco-elastic plastic model (VEP) which takes into account the effect of phase transformation-induced volumetric strain. Both models allowed highlighting the residual stresses evolution through the different zones. Numerical results showed a difference in the residual stresses distribution depending on the model used. In the end, it appears that the high temperature, specific to the laser beam, is the main factor governing the residual stresses. When comparing simulation results with experimental data, the values converge well in some zones, in particular the FZ and the others less.

Nondestructive separation of residual stress and composition gradients in thin films by angle- and energy-dispersive X-ray diffraction. II. Experimental validation

The different approaches for separating residual stress and composition gradients introduced in the first part of this series [Klaus &amp; Genzel (2017). J. Appl. Cryst. 50, 252–264] are demonstrated with the experimental example of a graded Ti(C,N) coating layer deposited by a modified high-temperature chemical vapour deposition process on a cemented carbide substrate. The coating layer features a depth gradient in the lattice parameter d_{\psi ^*}^{hkl} in the strain-free direction of the biaxial stress state, ψ*,hkl , and tensile residual stresses σ|| which are nearly uniform over the coating thickness but drop down significantly towards the free surface. On the assumption that the out-of-plane stress component σ33 can be neglected, the d_{\psi ^*}^{hkl} gradient is related to the variation in the chemical composition with depth. Therefore, the example considered here corresponds to case (d) of possible combinations of residual stress and composition gradients discussed in the first part of this series. The comparison of the results achieved by means of the different methods reveals the importance of choosing appropriate experimental conditions that fit best to the sample to be investigated. For the case of thin-film analysis, it is shown that the X-ray information depth is the crucial parameter which should match the film thickness.

Nondestructive separation of residual stress and composition gradients in thin films by angle- and energy-dispersive X-ray diffraction. I. Theoretical concepts

Different X-ray measurement and data evaluation concepts are presented, which allow for residual stress analysis in thin films with pronounced gradients in chemical composition. These gradients lead to a variation in the strain-free lattice parameter a 0 with respect to the film thickness and superimpose the lattice strain induced by the film's inherent stresses. Non-consideration of a 0(z) gradients is shown to lead to considerable errors (`ghost stresses') in the residual stress depth profiles. With the simulated example of a TiC x N1−x film with a pronounced carbon gradient, the first part of this series introduces four approaches, which permit the separation of residual stress and composition depth distributions at different levels of approximation. They are based on lattice spacing depth profile measurements performed in either the sin2ψ mode or the scattering vector mode, or in combinations of these two scanning modes. Depending on the approach used for separating the residual stress and composition gradients, angle- or energy-dispersive diffraction has to be applied, employing monochromatic X-ray sources available in the laboratory or either white high-energy synchrotron radiation or the Bremsstrahlung of conventional X-ray tubes, respectively. The methods introduced here assume a biaxial residual stress state within the film. For a triaxial residual stress state with σ33 ≠ 0, a separation of stress and composition gradients is not straightforward, because an a 0(z) gradient cannot be distinguished from the hydrostatic part of the stress tensor.

Influence of surface integrity on the fatigue behaviour of a hot-forged and shot-peened C70 steel component

Hot-forging is a process commonly used in the manufacturing of automobile parts such as connecting rods. Most hot-forged components are shot-blasted after forging in order to clean off the forging scale. The shot-blasting process is akin to shot-peening and greatly affects the surface integrity of the components by introducing hardening and residual stresses. This study focuses on the influence of the surface integrity on the fatigue behaviour of a hot-forged C70 steel connecting rod. Specimens were machined out of the connecting rods in order to perform fatigue testing on the forged surface. Several surfaces states, including shot-peened surfaces, were studied in order to quantify the influence of the surface integrity.A thorough characterisation of the surface integrity of the specimens was first performed. The hardness, residual stresses and microstructure gradients were analysed, and the surface of each specimen was scanned using a profilometer. The specimens show complex networks of surface defects introduced during forging, and the shot-blasting process introduces important surface residual stress and microstructure gradients. High cycle fatigue tests in plane bending were then performed on the specimens, with the surface scans helping to identify the critical defect on which crack initiation occurred. The fatigue results, presented in the form of a Kitagawa diagram, are analysed in order to determine which surface integrity parameters are the most influential on fatigue behaviour. The forging defects have an important negative effect on fatigue strength. After shot-blasting, the fatigue strength increases considerably because of the large compressive residual stresses introduced by the shot-blasting process.

Stress and thermal characterization of 4H-SiC microelectromechanical structures

In this letter, the potential of single crystalline 4H-polytype silicon carbide (4H-SiC) based microelectromechanical structures as resistance thermometer for high temperature sensing were explored. A dopant-selective photoelectrochemical etching process was applied to release the sensing element – suspended microstructures on 4H-SiC substrate. Residual stress and stress gradient in the microstructure before and after release was examined by micro-Raman spectroscopy. Electrical resistance of the suspended microstructures at different temperatures were characterized and analyzed by a temperature-dependent electron mobility model.

A study of machined surface RS gradient distribution and cumulative on HSM ball end milling

Residual stress (RS) has been intensively studied as an important criterion for evaluation of the surface integrity of high-speed machining (HSM) in this article. Based on the elastoplastic material property, a valuable machining property of gradient distribution of the machining path to the machined surface with HSM ball end milling is observed via an experimental test. Furthermore, an experimental formula is deduced from HSM ball end milling on H13 die steel. A cumulative coefficient for this effect is obtained by the residual stress measurement data. This is one method of predicting the residual stress concentration zone using the formula and coefficient to calculate the residual stress along the machining path. Based on the result, a discussion is presented in different fields of elastoplasticity metal material.

High quality factor RF MEMS tunable capacitor

This paper presents the design of a tunable capacitor that integrates two vertical comb-drive tunable capacitors and a parallel plate tunable capacitor. The parallel plate tunable capacitor is in the middle of the comb-drive tunable capacitors. The vertical comb-drive part has sets of fixed and moving fingers while the parallel plate has fixed and moving plates. The capacitor is fabricated using the MetalMUMPs microfabrication process, which has only one thick structural layer i.e. metal (20 μm nickel and 0.5 μm gold). The nickel of the metal layer is subject to residual stress gradients along its thickness. After release of the metal layer, the stress gradients bend the curve-up beams to raise the moving fingers and the moving plate of the capacitor above the substrate and the fixed plate. Hence, one structural layer (i.e. the metal) is used to form the moving and the fixed comb fingers without requiring two structural layers. Vertical comb-driven tunable capacitors offer high capacitance density when a large number of fingers with narrow gap are used. Therefore, high capacitance ratio and high quality factors can be achieved by using vertical comb-drives with in a relatively small device area compared to lateral comb-drives. The parallel plate driving part increases the displacement and capacitance ratio while reducing the actuation voltage required for driving the capacitor. The quality factor and tuning ratio of the fabricated tunable capacitor are 118.5 and 101% at 0.8 GHz; respectively at a driving voltage of 100 V. Based on the experimental results of the fabricated tunable capacitor, an optimized design is presented for both high tuning ratio and high quality factor. The tuning ratio of the optimized capacitor is found to be 143.1% at a “pull-in” displacement of 9.5 µm and a “pull-in” voltage of 90 V. The quality factors of the optimized capacitor are 520 at 0 V and 363.5 at 90 V, respectively at 0.8 GHz. The tunable capacitor achieves a displacement of more than one-half of the gap between the parallel plates without “pull-in” effect.

Evolution of Voids in Mg/Al Diffusion Bonding Process

AbstractThe closure process of voids was very crucial for diffusion bonding, voids behaviors in Mg/Al bonding process were investigated, and the control mechanisms of diffusion at different predominant process parameters were discussed in this paper. Finite element simulation was utilized to investigate the influence of thermal residual stresses on the appearance of secondary voids. The results showed that: the dominant mechanism of void closure was plastic deformation in the initial stage of Mg/Al diffusion bonding. Numerical results indicated that secondary voids could be easily generated in the regions where tensile residual stress gradient achieves the maximum, corresponding to area that Al3Mg2 layer at the nearby Al/Al3Mg2 interface, the segregation of voids deteriorated the performances of the bonded joints.

A translation micromirror with large quasi-static displacement and high surface quality

A large displacement with high surface quality translation micromirror is presented. The micromirror consists of a magnetic actuator and a mirror plate. The actuator and the mirror plate are fabricated separately using two processes and then bonded together. The actuator consists of a moving film which is a 20 µm thick nickel film fabricated by MetalMUMPs and a solenoid located underneath the moving film. The moving film is designed to curve up through the residual stress gradient in the nickel film and a curve-up mechanism which includes four trapezoidal plates and anchoring springs. The mirror plate is simply diced from a polished silicon wafer and coated with a metal thin film. The mirror plate is bonded onto the central ring of the moving film. A solenoid attracts the moving film along with the mirror plate downwards to realize translation. A quasi-static displacement of 123 µm is achieved at a driving current of 400 mA. A high mirror surface quality is realized, e.g. 15.6 m of curvature radius and 2 nm surface roughness.

Numerical simulation of residual stress in low temperature colossal carburised layer on austenitic stainless steel

A numerical model is proposed to quantitatively characterise the residual stress evolutions in low temperature colossal carburised layer on austenitic stainless steel. In this model, on the basis of the consideration of concentration and stress dependent carbon diffusivity, prediction of the carbon concentration distribution and growth regularity of carburised layer is performed. The strain rate is discussed taking the compositive effects of residual stress and carbon concentration gradient. Based on the strain compatibility of carburised layer and substrate, the residual stress is calculated. Meanwhile, a low temperature colossal carburisation experiment is carried out on 316L austenitic stainless steel and the carbon concentration and residual stress are measured to verify the validity of the model. The numerical results of carbon concentration and residual stress distributions agree with the experimental data, indicating that the numerical model established in this paper can be used to investigate the process of low temperature colossal carburisation.

Three-dimensional local residual stress and orientation gradients near graphite nodules in ductile cast iron

A synchrotron technique, differential aperture X-ray microscopy (DAXM), has been applied to characterize the microstructure and analyze the local mesoscale residual elastic strain fields around graphite nodules embedded in ferrite matrix grains in ductile cast iron. Compressive residual elastic strains are measured with a maximum strain of ∼6.5–8 × 10−4 near the graphite nodules extending into the matrix about 20 μm, where the elastic strain is near zero. The experimental data are compared with a strain gradient calculated by a finite element model, and good accord has been found but with a significant overprediction of the maximum strain. This is discussed in terms of stress relaxation during cooling or during storage by plastic deformation of the nodule, the matrix or both. Relaxation by plastic deformation of the ferrite is demonstrated by the formation of low energy dislocation cell structure also quantified by the DAXM technique.